Among various power semiconductor devices, a power MOSFET (field-effect transistor of a metal-oxide-semiconductor structure) as one kind of MISFET is known for a relatively low power loss and high-speed switching. The MOSFET, however, is desired to have a reduced ON-state resistance, since this element having a single kind of carriers (electrons or holes) performs no modulation of its conductivity by introducing minority carriers. On the other hand, a technique for forming trenches in a surface of a semiconductor element finds various applications, for example, to reduce the ON-state resistance of the semiconductor element. Thus, various structures of semiconductor elements having trenches have been proposed in recent years.
FIG. 7(a) is a cross sectional view showing a principal part of a conventional vertical MOSFET. This figure shows a unit cell of the MOSFET. In the actual MOSFET, this unit cell is repeatedly reversed with respect to the vertical line such that a multiplicity of unit cells are connected in series with each other. While this figure only shows an active region of the transistor assigned to perform switching of electric current, the actual semiconductor element needs to be provided with a peripheral portion which mainly contributes to withstanding voltage. The peripheral portion will not be described in detail since this portion is constructed in a normal form. In FIG. 7(a), an n drain drift region 702 in the form of an n epitaxial layer is superposed on an n.sup.+ substrate 701, to provide a semiconductor substrate. A p base region 703 is formed in a selected area of a surface layer of the semiconductor substrate, and an n.sup.+ source region 704 is formed in a part of a surface layer of the p base region 703. A gate electrode 707 is formed, through a gate oxide film 706, on the surface of the p base region 703 interposed between exposed surface areas of the n.sup.+ source region 704 and the n drain drift region 702. A source electrode 708 is formed in contact with both the n.sup.+ source region 704 and the p base region 703, and a drain electrode 709 is formed on the rear surface of the n.sup.+ substrate 701. In operation of this semiconductor element, when a positive voltage is applied to the gate electrode 707, an n-type inversion channel appears in a surface layer of the p base region 703 right below the gate electrode 707, whereby the n.sup.+ source region 704 is conducted with the n drain drift region 702. When the transistor is in its OFF state in which the gate voltage is not higher than a threshold voltage, the n-type inversion channel does not appear in the surface of the p base region 703. In this state, therefore, the voltage applied to the transistor is carried by a depletion layer which expands over both sides of a pn junction between the p base region 703 and the n drain drift region 702.
In the power MOSFET, several millions of unit cells each having the structure of FIG. 7(a) are integrated within one chip, so as to reduce the ON-state resistance. The ON-state resistance per unit area (Ron*A) and the withstand voltage are used as parameters for evaluating the performance of the power MOSFET. Where the withstand voltage is constant, the size of the chip is reduced with reduction of the ON-state resistance (Ron*A), so that the transistor can be manufactured at a reduced cost.
FIG. 7(b) is a view explaining details of the ON-state resistance of the power MOSFET of FIG. 7(a). The ON-state resistance of this transistor is a sum of a contact resistance (Rcnt) at an interface between the source electrode 708 and the n.sup.+ source region 704, a channel resistance (Rch) in the channel formed in the surface layer of the p base layer right below the gate electrode 707, a JFET resistance (Rjfet) caused by narrowing of a current path due to the depletion layer, and a resistance (Rdrift) in the n drain drift region 702.
In particular, the specific resistance and thickness of the n drain drift region 702 are important parameters for determining the withstand voltage of the element and the resistance (Rdrift) of the drift region 702. In the structure shown in FIG. 7(a), the optimum specific resistance and thickness of the n drain drift region 702 are determined depending upon the required level of the withstand voltage of the element, as described in A. S. Grove: Physics and Technology of Semiconductor Devices, John Wiley & Sons, p.197, FIG. 6.31, for example. Where the element is required to have a withstand voltage of 60 V, the specific resistance and thickness of the n drain drift region 702 are 0.8 .OMEGA..multidot.cm and 6.5 .mu.m, respectively, and an effective thickness of the n epitaxial layer (Weff) that determines the withstand voltage is about 6 .mu.m. The element withstand voltage, which is mainly determined by the structure as observed in the direction of depth of the element, is approximately equal to the withstand voltage of a diode including the p base region, n drain drift region 702 and the n.sup.+ substrate 701 that are arranged in the depth direction of the element.
FIG. 8(a) is a cross sectional view showing a principal part of another conventional MOSFET. This figure, like FIG. 7(a), shows a unit cell of the MOSFET. In this unit cell, an n drain drift region (n epitaxial layer) 802 is laminated on an n.sup.+ substrate 801, to provide a semiconductor substrate. A p base layer 803 is formed in a surface layer of the semiconductor substrate, and a trench 805 is formed from the surface of the p base layer 803 to reach the n drain drift region 802. An n.sup.+ source region 804 is formed in a part of a surface layer of the p base layer 803. A gate electrode 807 is disposed in the trench 805, with a gate oxide film 806 interposed therebetween. A source electrode 808 is formed in contact with both the n.sup.+ source region 804 and the p base region 803, and a drain electrode 809 is formed on the rear surface of the n.sup.+ substrate 801. In operation of this element, when a positive voltage is applied to the gate electrode 807, an n type inversion channel appears in a surface layer of the p base layer 803 beside the gate electrode 807, whereby the n.sup.+ source region 804 is conducted with the n drain drift region 802. When the transistor is in its OFF state in which the gate voltage is not higher than a threshold level, on the other hand, the inversion channel is not formed in the surface of the p base layer 803. In this state, the voltage applied to the transistor is carried by a depletion layer expanding over both sides of a pn junction between the p base layer 803 and the n drain drift region 802.
FIG. 8(b) is a view explaining details of the ON-state resistance of the power MOSFET of FIG. 8(a). The ON-state resistance of this transistor is a sum of a contact resistance (Rcnt) at an interface between the source electrode 808 and the n.sup.+ source region 804, a channel resistance (Rch) in the channel formed in the surface layer of the p base layer 803 which faces the gate electrode 807, and a resistance (Rdrift) in the n drain drift region 802. Due to the absence of the resistance (Rjfet) in the MOSFET of FIG. 8(a) having the trench 805, the ON-state resistance of this MOSFET can be reduced as compared with that of the first conventional example of FIG. 7(a). Further, since the inversion channel is formed in the vertical direction of the MOSFET of FIG. 8(a), the density of cells integrated in one chip (element) can be increased, with a result of reduction of the ON-state resistance, as compared with the first conventional example of FIG. 7(a).
When the MOSFET of FIG. 8(a) is required to have a withstand voltage of 60 V, for example, the specific resistance of the n drain drift region 802 is 0.8 .OMEGA..multidot.cm, and the thickness thereof is 6.5 .mu.m, as in the first conventional example of FIG. 7(a). The effective thickness of the epitaxial layer (Weff) that determines the withstand voltage is about 6 .mu.m.
In the structure of the first conventional example of FIG. 7(a), the size of the unit cell has been significantly reduced owing to a fine working or processing technique developed in recent years, and the resistances Rcnt, Rch and Rjfet of the resulting semiconductor element have been significantly reduced. At present, the ON-state resistance (Ron*A) of the power MOSFET having the withstand voltage of 60 V is 1.5 m .OMEGA..multidot.cm.sup.2, and the resistance (Rdrift) of the n drain drift region 702 amounts to about one third of the ON-state resistance, i.e., about 0.5 m.OMEGA..multidot.cm.sup.2. This resistance Rdrift, however, cannot be reduced due to the structure of the conventional element.
With respect to the structure of the second conventional example of FIG. 8(a), the ON-state resistance (Ron*A) is 1.0 m.OMEGA..multidot.cm.sup.2, and the resistance (Rdrift) of the n drain drift region 802 amounts to about a half of the ON-state resistance, i.e., about 0.5 .OMEGA..multidot.cm.sup.2.
Even with a further development of the fine working technique in the future, this resistance (Rdrift) of the epitaxial layer, that is, the n drain drift layer, will not be reduced as long as the power MOSFET has one of the conventional structures as described above.